Sunao Tsuboi on Miyuki Bridge, where he was photographed three hours after the bombing of Hiroshima.
Photograph: Alamy

It is not as if Sunao Tsuboi needs another reminder of his violent encounter, as a 20-year-old university student, with a “living hell on earth”. The facial scars he has carried for seven decades are proof enough. But, as if to remind himself of the day he became a witness to the horrors of nuclear warfare, he removes a a black-and-white photograph and points to the shaved head of a young man looking away from the lens.

“That’s me,” he says. “We were hoping we would find some sort of medical help, but there was no treatment available, and no food or water. I thought I had reached the end.”

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The location is Miyuki Bridge, Hiroshima, three hours after the Enola Gay, a US B-29 bomber, dropped a 15-kiloton nuclear bomb on the city on the morning of 6 August 1945. Between 60,000 and 80,000 people were killed instantly; in the months that followed the death toll rose to 140,000.

In the photo, one of only a handful of surviving images taken in Hiroshima that day, Tsuboi is sitting on the road with several other people, their gaze directed at the gutted buildings around them. To one side, police officers douse schoolchildren with cooking oil to help soothe the pain of their burns.

As Japan prepares to mark the 70th anniversary of the first nuclear attack in history, Tsuboi and tens of thousands of other hibakusha (atomic bomb survivors) are again confronting their own mortality.

“People like me are losing the strength to talk about their experiences and continue the campaign against nuclear weapons,” says Tsuboi, a retired school principal who has travelled the world to warn of the horrors of nuclear warfare.

The average age of the 183,000 registered survivors of the Hiroshima and Nagasaki attacks rose just above 80 for the first time last month.

While each has a unique recollection of the morning of 6 August and its aftermath, near disbelief at the scale of destruction is a theme that runs through hibakusha testimony.

Tsuboi remembers hearing a loud bang, then being blown into the air and landing 10 metres away. He regained consciousness to find he had been burned over most of his body, his shirtsleeves and trouser legs ripped off by the force of the blast.

“My arms were badly burned and there seemed to be something dripping from my fingertips,” said Tsuboi, who is co-chair of Nihon Hidankyo, a nationwide organisation of atomic and hydrogen bomb sufferers.

Tsuboi indicates on a map where he was when the city was attacked on the morning of 6 August 1945. Photograph: Justin McCurry/Guardian

“My back was incredibly painful, but I had no idea what had just happened. I assumed I had been close to a very large conventional bomb. I had no idea it was a nuclear bomb and that I’d been exposed to radiation. There was so much smoke in the air that you could barely see 100 metres ahead, but what I did see convinced me that I had entered a living hell on earth.

“There were people crying out for help, calling after members of their family. I saw a schoolgirl with her eye hanging out of its socket. People looked like ghosts, bleeding and trying to walk before collapsing. Some had lost limbs.

“There were charred bodies everywhere, including in the river. I looked down and saw a man clutching a hole in his stomach, trying to stop his organs from spilling out. The smell of burning flesh was overpowering.”

He was taken to a hospital, where he remained unconscious for over a month. By the time he came to, a defeated Japan was under the control of the US-led allied occupation. “I had no idea that the war had ended,” he said. “It was difficult to take in.”

Since then Tsuboi has been hospitalised 11 times, including three occasions when doctors told him he was about to die. He takes drugs for several illnesses, including two cancer diagnoses, which he says are connected to his exposure to radiation.

While the A-bomb survivors’ testimony is now a matter of historical record, the hibakusha are trying to ensure that their experiences don’t die with them, at a time when the world is facing nuclear threats from North Korea and Russia.

Earlier this year one of the most active branches of Hidankyo announced it would disband after its members, most of whom are in their 80s and 90s, conceded they were too old to continue their activities.

“In 10 years, I’d be surprised if there are many of us left,” says Hiroshi Shimizu, a Hidankyo official who was three years old when the Hiroshima bomb exploded a mile (1.6km) from his home.

“If the hibakusha continue to speak out against nuclear weapons, then other people will follow suit. That’s why we have to continue our campaign for as long as we are physically able.”

Hiroko Hatakeyama, a survivor of the Hiroshima atomic bombing, looks at a family photograph taken before the attack. Photograph: Justin McCurry/Guardian

Hiroshima and Kunitachi, a small city in western Tokyo with a small population of A-bomb survivors, have tried to preserve the hibakusha legacy by setting up “storyteller” courses open to people who have no direct experience of the attacks and no A-bomb survivors among their relatives. Hidankyo, meanwhile, has started reaching out to the children and grandchildren of hibakusha.

Last month, Yoshiko Kajimoto, an 84-year-old survivor, recounted her experiences via Skype to dozens of members of the British parliament, and a delegation of hibakusha recently took part in a 24-country “voyage for peace” with the Japanese NGO Peace Boat.

As of August 2014, the number of people recognised as having died from the effects of the two atomic bombs stood at more than 450,000: 292,325 in Hiroshima and 165,409 in Nagasaki, which was bombed three days later. On 15 August, Emperor Hirohito announced Japan’s surrender to a shattered nation.

“I won’t be here in 10 or 15 years’ time, so the question we’re all asking is how to continue sending our message,” said Hiroko Hatakeyama, who was six in 1945.

“I barely have the energy to campaign these days, and I’m no longer scared of dying. But at the same time I realise that it’s our duty as survivors to carry on for as long as possible, to honour the memory of those who are no longer with us.”

Tsuboi, who went on to have three children and seven grandchildren, will make his annual pilgrimage to Hiroshima Peace Park on 6 August. That evening, he will release a lantern along the Motoyasu river – where thousands fled to escape the heat of the nuclear blast – to “guide” the spirits of the dead.

Editorial: The president has long been drawn to the ideal of a world without atomic weapons. His trip to Japan will encourage the vision, but the gritty realpolitik of the region renders it a distant dream